1. Field
The disclosed concept pertains generally to vacuum circuit breakers and other types of vacuum switchgear and related components, such as vacuum interrupters and arc-resistant shields. In particular, the disclosed concept pertains to new alloy compositions for use in constructing internal arc-resistant shields employed in the vacuum interrupter chamber.
2. Background Information
Vacuum interrupters are typically used to interrupt high voltage AC currents. The interrupters include a generally cylindrical vacuum envelope surrounding a pair of coaxially aligned separable contact assemblies having opposing contact surfaces. The contact surfaces abut one another in a closed circuit position and are separated to open the circuit. Each electrode assembly is connected to a current carrying terminal post extending outside the vacuum envelope and connecting to an AC circuit.
An arc is typically formed between the contact surfaces when the contacts are moved apart to the open circuit position. The arcing continues until the current is interrupted. Metal from the contacts that is vaporized by the arc forms a neutral plasma during arcing and condenses back onto the contacts and also onto a vapor shield placed between the contact assemblies and the vacuum envelope after the current is extinguished.
The vacuum envelope of the interrupter typically includes a ceramic tubular insulating casing with a metal end cap or seal covering each end. The electrodes of the vacuum interrupter extend through the end caps into the vacuum envelope. At least one of the end caps is rigidly connected to the electrode and must be able to withstand relatively high dynamic forces during operation of the interrupter.
Vacuum interrupters are key components of vacuum-type switchgear. It is typical for interrupters for vacuum-type circuit breakers using transverse magnetic field contacts to include the vapor shield, e.g., internal arc shield or arc-resistant shield, that is resistant to heavy arcing to restrict the outward dissemination of the arc and preserve the high voltage withstand of the interrupter after breaking the fault current.
It is customary for the shield to be constructed of copper, stainless steel, copper-chromium alloy or a combination thereof. In some cases, the shield may be constructed of one material in the arcing area and a second material may be used for the remainder of the shield. The copper-chromium alloy material may be used for the highest fault current ratings because of its resistance to arc damage and its ability to hold off high voltages after the arcing has occurred. It is typical for the copper-chromium alloy to include about 10 to 25% by weight chromium and the balance copper.
It is an object of the disclosed concept to develop new alloy compositions for use in constructing arc-resistant shields for internal use in vacuum interrupters wherein the compositions are other than the conventional pure chromium and copper alloys. It is a further object to develop new alloy compositions wherein the amount of chromium is present in a reduced amount as compared to known copper-chromium compositions. In still a further object, chromium is absent from the compositions. Chromium is expensive to obtain and therefore, reducing or eliminating the presence of chromium will provide a lower cost alternative to the conventional materials used in constructing arc-resistant shields. Further, it is believed employing materials or elements other than pure chromium and copper can result in alloy compositions which exhibit superior performance in arc-resistant shields.